media: v4l2-pci-skeleton.c: fix doc warning

Document the 'field' field to fix this warning:

samples/v4l/v4l2-pci-skeleton.c:80: warning: Function parameter or member 'field' not described in 'skeleton'
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

samples/v4l/v4l2-pci-skeleton.c

@@ -58,6 +58,7 @@ MODULE_LICENSE("GPL v2");
  * @queue: vb2 video capture queue
  * @qlock: spinlock controlling access to buf_list and sequence
  * @buf_list: list of buffers queued for DMA
+ * @field: the field (TOP/BOTTOM/other) of the current buffer
  * @sequence: frame sequence counter
  */
 struct skeleton {